Interactive virtual environments are useful tools for hands-on learning or rehearsing of procedural tasks. However, task training applications typically provide a constrained course of action for the learner, forcing them down a single specific solution path. We discuss an approach in which the virtual environment is semantically instrumented in order to allow for the tracking of and reasoning about open-ended learner activity therein. Our approach leverages ontology-based knowledge representation which allows for a structured and meaningful description of virtual objects and of the learner actions that may be performed upon them. This is facilitated by the association of specific ontological classes with geometric components of the objects which populate a training exercise. These classes, together with their attributes, relationships and rules, characterize the environment and user actions in a readily understandable manner. As a result, a training system is able to observe the learner activity, render an assessment of that activity, and provide meaningful feedback to the learner. We also present an authoring tool which allows content developers to semantically annotate three-dimensional models for such an environment.